1. Field of the Invention
The present invention relates to superconducting magnets, and more particularly to a method and apparatus to enable an orderly run-down of the magnet in the case of failure of the power supply to the refrigerator.
In particular, the present invention relates to a method and apparatus that enable energy stored within the magnetic field of a superconducting magnet to be used to continue operation of a refrigerator, cooling superconducting current leads and the afore-mentioned superconducting magnet to below their transition temperature for long enough to ensure a controlled run-down of the current in the magnet, avoiding a quench and dissipating heat outside of the cryostat.
2. Description of the Prior Art
Typically, superconducting magnets are housed in a cryostat, which keeps the magnet below its transition temperature. While this was once achieved by providing a bath of liquid cryogen, more recent designs have the magnet in a vacuum, cooled by conduction over a thermal link to a cryogenic refrigerator. In such arrangements, is has become common to provide an electrical current lead from the magnet to an externally accessible terminal, of which at least part is formed of a high-temperature superconductor (HTS). In such arrangement, the refrigerator must be kept operating continuously, since thermal leakage into the cryostat will rapidly heat parts of the magnet and/or the HTS part of the current lead to above the superconducting transition temperature if refrigeration were to cease.
A problem therefore occurs with a refrigerator which is electrically powered (as is typical), in the case of failure of the electrical supply. As is well known, electric current continues to flow in a superconducting magnet, even in the absence of an applied voltage. The present invention seeks an orderly way of reducing this current before a quench initiates in the magnet or the HTS part of the current lead, which may be referred to below as the HTS current lead.
An alternative approach is to intentionally induce a quench, which is spread throughout the material of the magnet, so that no part of the magnet is raised to a temperature high enough to suffer damage. However, this approach still causes a large temperature rise to the magnet, leading to significant down-time while the magnet is re-cooled. The quench may also cause some movement of wires or coils in the magnet, which may mean that a time-consuming re-shimming process needs to be carried out.